The surface reaction model for the mechanism of mass transfer-metabolism in the fixed-film nitrification process is investigated and discussed for the design of trickling filter systems and rotating biological contactor (RBC) systems. Two experimental studies are performed. The first study, using a stationary fixed-film reactor to simulate the trickling filter process, reveals that the surface reaction kinetics follow a pseudo-homogeneous model. The second study, using a bench-scale RBC unit, indicates that the pseudo-homogeneous model is also applicable to the rotating fixed-film process. The effective slime thickness of an RBC system can be estimated from this model by locating the optimum NH3-N removal rotating speed and finding the corresponding liquid film thickness at that rotating speed.